Добірка наукової літератури з теми "Kimban Orogeny"

Mesoproterozoic felsic magmatism of the Gawler Range Volcanics and Hiltaba Suite granites occurred at 1585–1595 Ma across much of the Gawler Craton, South Australia. Nd isotopic analysis of this felsic magmatism, combined with petrological and geochemical arguments, suggest derivation by partial melting of both Paleoproterozoic and Archean crust. The majority of samples analyzed have Nd isotopic and geochemical characteristics compatible with the involvement of Paleoproterozoic crust stabilized during the 1.85–1.71 Ga Kimban orogeny as sources for the Mesoproterozoic magmatism; others require derivation from sources dominated by Archean rocks. This cycle of Paleoproterozoic crustal stabilization followed by involvement of this crust Mesoproterozoic felsic magmatism is one previously documented from many parts of Mesoproterozoic Laurentia. On the basis of models proposing East Australia–Antarctica to be the conjugate landmass at the rifted margin of western North America, it appears that the voluminous magmatism of South Australia is another example of a typically Mesoproterozoic style of magmatism linked to Laurentia. This Mesoproterozoic magmatism appears temporally linked to regional high-temperature, low-pressure metamorphism of the region, and together with the presence of mantle-derived magmas, implicates the operation of large-scale tectono-thermal processes in the origin of felsic magmatism at 1590 Ma.

Zirconium is an element of considerable petrogenetic significance but is rarely found in hematite at concentrations higher than a few parts-per-million (ppm). Coarse-grained hematite ore from the metamorphosed Peculiar Knob iron deposit, South Australia, contains anomalous concentrations of Zr and has been investigated using microanalytical techniques that can bridge the micron- to nanoscales to understand the distribution of Zr in the ore. Hematite displays textures attributable to annealing under conditions of high-grade metamorphism, deformation twins (r~85˚ to hematite elongation), relict magnetite and fields of sub-micron-wide inclusions of baddeleyite as conjugate needles with orientation at ~110˚/70˚. Skeletal and granoblastic zircon, containing only a few ppm U, are both present interstitial to hematite. Using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analysis and mapping, the concentration of Zr in hematite is determined to be ~260 ppm on average (up to 680 ppm). The Zr content is, however, directly attributable to nm-scale inclusions of baddeleyite pervasively distributed throughout the hematite rather than Zr in solid solution. Distinction between nm-scale inclusions and lattice-bound trace element substitutions cannot be made from LA-ICP-MS data alone and requires nanoscale characterization. Scandium-rich (up to 0.18 wt. % Sc2O3) cores in zircon are documented by microprobe analysis and mapping. Using high-angle annular dark field scanning transmission electron microscopy imaging (HAADF-STEM) and energy-dispersive spectrometry STEM mapping of foils prepared in-situ by focused ion beam methods, we identify [011]baddeleyite epitaxially intergrown with [22.1]hematite. Lattice vectors at 84–86˚ underpinning the epitaxial intergrowth orientation correspond to directions of r-twins but not to the orientation of the needles, which display a ~15˚ misfit. This is attributable to directions of trellis exsolutions in a precursor titanomagnetite. U–Pb dating of zircon gives a 206Pb/238U weighted mean age of 1741 ± 49 Ma (sensitive high-resolution ion microprobe U–Pb method). Based on the findings presented here, detrital titanomagnetite from erosion of mafic rocks is considered the most likely source for Zr, Ti, Cr and Sc. Whether such detrital horizons accumulated in a basin with chemical precipitation of Fe-minerals (banded iron formation) is debatable, but such Fe-rich sediments clearly included detrital horizons. Martitization during the diagenesis-supergene enrichment cycle was followed by high-grade metamorphism during the ~1.73–1.69 Ga Kimban Orogeny during which martite recrystallized as granoblastic hematite. Later interaction with hydrothermal fluids associated with ~1.6 Ga Hiltaba-granitoids led to W, Sn and Sb enrichment in the hematite. By reconstructing the evolution of the massive orebody at Peculiar Knob, we show how application of complimentary advanced microanalytical techniques, in-situ and on the same material but at different scales, provides critical constraints on ore-forming processes.

Regional gravity and magnetic anomalies that originate from crystalline basement rocks extend over many parts of the Western Canada Sedimentary Basin. Although these potential-field anomalies provide a basis for tectonic subdivisions of the basement crust, most previous interpretations of these features have been largely qualitative in nature. This study focuses on numerical simulation and quantitative interpretation of five regional potential-field anomalies in Alberta, Canada, for which independent constraints on crustal structure are available from Lithoprobe seismic and electromagnetic studies. The Kimiwan High (~55°N, 116°W) is a roughly 250 km long linear magnetic high. Seismic profiles across this anomaly provide evidence for a crustal-scale extensional fault system that offsets the Winagami reflection sequence, a series of mid-crustal sills. We find that the magnetic anomaly can be modelled as either a 15–40 km wide zone of moderate positive susceptibility (4.5 × 10–3 SI units) in the hanging wall of the detachment (5–17 km depth), or as a narrower (5–10 km), steeply dipping zone of high susceptibility (2.5 × 10–2 SI units) in the footwall (16–32 km depth). We interpret the former scenario as indicative of an extensive zone of alteration above the fault, whereas the latter could represent a decapitated granitic pluton that correlates with magmatic rocks farther north. To the southeast, the Thorsby Low (~53°N, 114°W) is a sinuous, 400 km long magnetic low and gravity gradient trend that appears to be a splay of the much more extensive Snowbird tectonic zone. Previous seismic interpretations across the Thorsby Low indicate that it coincides with a 10 km offset in the Moho. Our results show that this abrupt change in crustal thickness is consistent with, but not required by, the gravity signature of this feature. The northeast-striking Red Deer High (~53°N, 112°W) is a narrow magnetic anomaly in central Alberta with variable intensity along strike. Previous magnetotelluric studies suggest that the Red Deer anomaly is closely associated with a linear, highly conductive body in the upper basement. Our modelling results confirm the shallow depth of the causative body and suggest an eastward dip that is consistent with published seismic interpretations. Near the eastern border of Alberta, the 300 km long Eyehill High (~52°N, 110°W) is a prominent north-striking magnetic anomaly adjacent to the western hinterland of the Trans-Hudson Orogen. Combined gravity and magnetic modelling show that this feature occurs near the western boundary of a large block of dense material in the lower crust. The magnetic anomaly can be simulated by a near-vertical dyke-like body in the upper crust. Lastly, the Vulcan structure (~50.5°N, 112°W) forms the ~400 km long, northern boundary of the Archean Medicine Hat block. It is defined by a sinuous east-trending magnetic anomaly and gravity low. The wavelength and polarity of the magnetic anomaly (positive in the north), coupled with the coincident gravity low, are most simply explained by a mid-crustal low-density body with significant remanent magnetization oriented antiparallel to the present-day field.

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In situ monazite U-Pb dating from metasedimentary rocks in the core of the crustal scale Kalinjala Shear Zone in the eastern Gawler Craton indicates that peak condition of > 9 kbar at temperatures of around 810 ºC occurred at c. 1700 Ma during the craton-wide Kimban Orogeny. Detrital zircon ages in metasedimentary rocks that contain the peak metamorphic assemblages indicate that maximum depositional ages for rocks in the core of the shear zone were around 1780 Ma, indicating that sedimentation occurred in the interval c. 1780-1700 Ma. Metapelite contains an early assemblage preserved in garnet cores characterised by a kyanite-rutile association. The enclosing matrix contains a cordierite-bearing assemblage that formed during the development of the principle gneissic fabric within the shear zone and documents ~4 kbar of decompression of the shear zone core during deformation. Garnet-biotite diffusional modelling suggests that the shear zone core cooled > 50 ºCMyr-1 implying rapid exhumation of the core. Fe-mg garnet diffusional modelling suggests that on the flanks of the shear zone that exhumation and cooling rates were slower, and the maximum metamorphic pressures were less than in the core, suggesting that the central region of the Kalinjala Shear Zone was rapidly exhumed compared to the flanks of the shear zone. Where the shear zone reworks rocks belonging to the early Paleoproterozoic Carnot Gneiss, early formed high pressure, high temperature assemblages are overprinted by lower pressure granulite assemblages leading to the formation of secondary cordierite-spinel at the expense of garnet-sillimanite. In Mg-Al rich rocks these early assemblages include rare garnet-sillimanite-orthopyroxene assemblages which formed at the expense of early sapphirine-rutile bearing associations. The garnet-sillimanite-orthopyroxene assemblage has been overprinted by cordierite-spinel-sapphirine-biotite at c. 1745 Ma. This age is slightly older than typically assigned to the Kimban Orogeny, and suggests that the event may be longer lived than previously thought. The timing of the earlier high pressure assemblage is equivocal, and could conceivably be related to the previously recognised c. 2450 Ma high-grade metamorphism in the Carnot Gneiss, and therefore not part of the Kimbanaged metamorphic architecture. The metamorphic constraints and age data from the core of the Kalinjala Shear Zone, combined with existing data, support a transpressional setting associated with the Kimban Orogeny. No evidence was found to support previously proposed models that include an extensional setting, or a c. 1850 Ma evolution of the shear system.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 2011

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A major electrically conducting structure has been spatially located in the southern Eyre Peninsula, South Australia. The structure extends from the continental margin inland along the eastern margin of the Eyre Peninsula, trending north-northeast for approximately 150 km. In order to provide a two-dimensional image of the crust orthogonal to the conductor’s strike, 39 broadband (1000 to 0.01 Hz) magnetotelluric sites were collected with approximately 2 km separation across the peninsula. A smoothed 2-D inversion model demonstrated that the conductor appears centred beneath a topographic high, structurally bound at the east by the transpressional Kalinjala Shear Zone and resistive Donington Suite granitoids, and the Sleaford Complex to the west. The main features from modelling are: (i) east of the Kalinjala Shear Zone, a region of high resistivity (> 1000 ohm/m) relates to the Donington Suite granitoids; (ii) the late Archaean Sleaford Complex (2480–2420 Ma) bordering the Donington Suite granitoids features a lower, wider resistivity range between 5 to < 600 ohm/m, and is near-vertical in the top 12 km; (iii) the lowest resistivity structure of < 0.1 ohm/m occurs at a depth of 5-10 km, and appears to terminate at a depth of ~15 km; (iv) the low resistivity structure correlates with banded iron formations and is credibly the result of biogenically deposited graphite in marine sediments, which migrated to become concentrated in fold hinges during the Kimban Orogeny; and (iv) the conductor is co-located with a ridge of high gravity (+ 200 to 500 mGals). The origin of this high gravity may be due to a mafic intrusive block of oceanic crust, compressed during the continental collision of the Kimban Orogeny. Utilising the constraints of the 2-D model, a regional 3-D forward model was developed which shows agreement with compiled legacy data sets.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2018